It is well known that the structure of peripheral nervous system myelin differs from that of the central nervous system in: (a) the satellite cells that produce the myelin sheaths; (b) the radial structural period of the concentric cylindrical lamellar arrays; (c) the degree of symmetry of the individual membranes of the membrane-pair units; (d) the relative amounts of three kinds of protein and of various lipids present; and (e) the relative affinities for insertion of water between the external appositions of the membrane-pairs to cause swelling. In general, central myelin is more compact, its membranes are more nearly symmetrical, and swelling is not readily achieved, as compared to peripheral myelin which may be normally slightly swollen and readily accepts aqueous layers between the membrane-pairs. Since relatively few kinds of nerve have been studied to date, there is the appearance in existing information that some of the central- peripheral differences may be discrete rather than continuous over the nervous system. This project proposes to perform a neuroanatomical survey of these factors in human nerves, featuring the use of small- angle x-ray diffraction. Hypothetical models for distribution of the various proteins and lipids, with their attendant net cationic and anionic charges, have been developed against which to interpret the results. Artificial alterations of myelin composition and structure will be attempted, and abnormal pathological myelins will be studied in relation to the models of normal structure. Factors influencing swelling will be systematically investigated. Several examples of myelinated nerves phylogenetically removed from mammalian cases will also be explored (fishes, earthworms and shrimps).